Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to hydraulic systems for operating mechanical members, such as booms of agricultural, construction and industrial equipment; and particularly to operating the hydraulic system in an emergency, such as when power to a hydraulic pump of the equipment is lost.
2. Description of the Related Art
Industrial equipment, such as lift trucks, have moveable members which are operated by hydraulic cylinder and piston arrangements. Application of hydraulic fluid to the cylinder traditionally has been controlled by a manual valve, such as the one described in U.S. Pat. No. 5,579,642. A manual operator lever was mechanically connected to move a spool within the valve. Movement of the spool into various positions with respect to cavities in the valve body enables pressurized hydraulic fluid to flow from a pump to one of the cylinder chambers and be drained from another cylinder chamber. The rate of flow into the associated chamber is varied by varying the degree to which the spool is moved, thereby moving the piston at proportionally different speeds.
Because the manual valves are mounted in or near the operator cab of the equipment, individual hydraulic lines have to be run from the valve to the associated cylinders. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near the operator cab. Instead, the solenoid valves are mounted adjacent the associated cylinders, thereby requiring that only a hydraulic line from the pump and another line back to the fluid tank need to be run through the equipment. Although electrical signals have to be transmitted from the operator cab to the solenoid valves, wires are easier to run and less prone to failure than pressurized hydraulic lines that must be flexible to accommodate movement of the equipment.
Industrial lift trucks require that the boom be capable of being lowered in a controlled manner should the engine fail thus removing power that drives the hydraulic pump. A simple way to provide this capability is to incorporate a valve that releases the hydraulic fluid in the boom cylinder, thereby enabling the boom to descend under the force of gravity. However, a load carrier is pivotally attached to the boom in many types of equipment and simply lowering the boom will cause the load carrier to tilt downward and allow a load to fall off. Thus even in an emergency, hydraulic power must be applied to a load carrier cylinder to maintain the load carrier level as the boom lowers. A previous solution was to incorporate a hand-operated emergency pump that supplied pressurized fluid to the cylinder that pivoted the load carrier with respect to the descending boom.
The present invention provides a method for operating hydraulic actuators on a machine in a controlled manner upon failure of the source of pressurized fluid that normally powers the actuators. The method is particularly useful to lower a boom of the machine that is operated by a first hydraulic actuator. A load carrier, pivotally coupled to the boom, is operated by a second hydraulic actuator.
During a failure of the hydraulic power source, fluid can be drained under pressure from the first hydraulic actuator, thereby enabling the boom to descend under the force of gravity. The draining hydraulic fluid is conveyed from the first hydraulic actuator to the second hydraulic actuator to produce movement of the load carrier with respect to the boom. The flow of the hydraulic fluid into the second hydraulic actuator is controlled so that as the boom moves, the angular relationship of the load carrier with respect to a support surface on which the machine rests is maintained substantially constant. For example, during descent the angle between the boom and the support surface changes. The change is measured and the flow of the hydraulic fluid is controlled to alter load carrier""s position with respect to the boom so that the load carrier remains level.
In one embodiment, sensors indicate the positions of the boom and the load carrier. For example a first angle between the boom and a carriage of the machine is sensed and a second angle between the boom and the load carrier is sensed. As the first angle changes, the hydraulic fluid flow into the second actuator is controlled to produce an equivalent change of the second angle of the load carrier. An amount of hydraulic fluid that is drained from the first actuator in excess of that required to operate the actuators is conveyed to a reservoir for the hydraulic system of the machine.
In another embodiment an inclinometer is attached to the load carrier to detect the angle of tilt with respect to the horizontal. In this version the flow of fluid to the second actuator is controlled to maintain the inclination of the load carrier substantially constant.